1. Field of the Invention
This invention relates to a wireless communication system, more particularly to a MIMO (multiple input multiple output) wireless communication system.
2. Description of the Related Art
The technology of MIMO has been applied to a variety of wireless communication systems recently, including the WLAN (wireless local area network), the WiMAX (Worldwide Interoperability for Microwave Access) and the 4G (4th generation) mobile phone system. These systems have adopted the MIMO to increase transmission speed or channel capacity.
Inevitably, additional antennas and RFE (radio frequency front-end) and AFE (analog front-end) circuits are necessary at the transmitting and receiving ends of the corresponding transmission routes of a MIMO. The hardship of the current application with MIMO is however, to reduce power consumption of the additional components that are present in the corresponding transmission routes.
The general format of each data frame or packet utilized in a wireless system is illustrated in FIG. 1. The format usually includes a preamble sequence used for frame or packet detection, AGC (automatic gain control), carrier synchronization, and timing synchronization. Certain systems have pilot sequences that are placed within the preamble sequence or in between data sequences for adaptive tracking in channel estimation or equalization, and for timing and frequency tracking.
An additional header sequence is usually required in front of the data sequence in cases when the modulation scheme, coding rate, or number of spatial streams is modified for different demands of channel quality and throughput so that the receiver can demodulate and decode the subsequent data sequences correctly. The header sequence is generally encoded with the modulation scheme and coding rate having the lowest level SNR (signal-to-noise ratio) requirements. Hence, the header sequence can be more immune to poor channel responses.
FIG. 2 depicts the conventional multiple-antenna transmitter in block diagram. As illustrated, a multiple-antenna transmitter 200 includes a TX (transmit) frame controller 210, a sequence selection circuit 212, a MIMO modulation and coding circuit 214, and a number of M TX RFE and AFE circuits 216_1˜216_M. When the transmitter transmits a data frame or packet, the TX frame controller 210 sends the preamble sequence, the header sequence, and data sequence consecutively to the MIMO modulation and coding circuit 214 via the sequence selection circuit 212 during a certain period of time. The MIMO modulation and coding circuit 214 modulates and encodes the sequences according to a specific modulation scheme, a specific coding rate, and a specific number of spatial streams and then transmits the signals of each transmitting route from the M antennas through the TX RFE and AFE circuits 216_1˜216_M, wherein M is an integer greater than or equal to 1.
In order to increase transmission speed or channel capacity in different requirements of channel quality and throughput, the data sequences are modulated and encoded according to various modulation schemes, coding rates, or number of spatial streams such that the signals transmitting via the transmission routes need not be the same all the time. Parameters of the data sequences are placed in the contents of the header sequence to inform the receiver about relevant information for demodulation and decoding. It should be noted that the header sequence is modulated and encoded according to the lowest level SNR requirements to ensure successful demodulation and decoding at the receiving end.
FIG. 3 depicts the conventional multiple-antenna receiver in block diagram. As illustrated, a multiple-antenna receiver 300 includes a RX frame controller 310, a MIMO demodulation and decoding circuit 312, a sequence separation circuit 314, and a number of N RX RFE and AFE circuits 216_1˜216_N, wherein N is an integer greater than or equal to 1. The receiver collects signals from N receiving routes that corresponds to N antennas and N RX RFE and AFE circuits 216_1˜216_N. A preamble sequence received is processed for frame or packet detection to track the arrival of a data frame or packet. Processes including the AGC, carrier synchronization, timing synchronization, and frame synchronization are then handled when the data frame or packet is detected. The timing of the subsequent sequences is generally confirmed after the frame synchronization process. The RX frame controller 310 demodulates and decodes the header sequence during a corresponding time interval and applies MIMO demodulation and decoding using the lowest level SNR requirements. Information extracted from the header sequence during the time interval determines the schemes for further MIMO demodulation and decoding processes applied to subsequent data sequences.
Nonetheless, increased power consumption may be a problem in multiple-antenna systems because of the multiple transmission routes. The actual power consumption (PRX) should be calculated by averaging the transmit power consumption (PTX), the receive power consumption (PRX), the idle power consumption (PRX—Idle), and the sleep power consumption (PSleep) by weighting them with their respective operating time. The formula of the weighted average is set forth as follows:
      P    Avg    =                              P          TX                ·                  T          TX                    +                        P          RX                ·                  T          RX                    +                        P          RX_Idle                ·                  T          RX_Idle                    +                        P          Sleep                ·                  T          Sleep                                    T        TX            +              T        RX            +              T        RX_Idle            +              T        Sleep            
Hence, important issues have been raised on topics regarding efficient ways to cut down transmit power consumption, receive power consumption, idle power consumption, and sleep power consumption when the time interval for each operation is preset.
The transmitting and receiving tasks carried out in multiple-antenna systems are usually the most power consuming in various aspects of system application. The required number of transmitting and receiving antennas should be at least the number of spatial streams utilized for transmitting and receiving data signals. Comparatively, it is easier for the transmitter to decide which of the specific antennas are to be used for transmitting data because the transmitting end has full knowledge initially of the spatial streams necessary for data sequence transmission. On the other hand, it is not until the header sequence is demodulated and decoded that the receiver gets an idea of the number of spatial streams that should be used for receiving the data sequences. Hence, the typical approach is to turn on all receiving antennas initially when entering a receive mode and to make use of the spatial diversity gain of multiple antenna systems during the process of packet receiving to enhance receiver efficiency.
Accordingly, the tradeoff between MIMO receiving efficiency and average receiving power consumption is an issue yet to be solved for wireless communication.